Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus which irradiate a thin plate-like precision electronic substrate (hereinafter referred to simply as a “substrate”) of silicon or silicon-germanium such as a semiconductor wafer with a flash of light to form a silicide or a germanide.
Description of the Background Art
In the process of manufacturing a semiconductor device, attention has been given to flash lamp annealing (FLA) which heats a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate a surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer.
Such flash lamp annealing is used for processes that require heating in an extremely short time, for example, typically for the activation of impurities implanted in a semiconductor wafer. The irradiation of a surface of a semiconductor wafer implanted with impurities by an ion implantation process with a flash of light from flash lamps allows the temperature rise to an activation temperature only in the surface of the semiconductor wafer in an extremely short time, thereby achieving only the activation of impurities without deep diffusion of the impurities.
It has also been under consideration to apply the flash lamp annealing to the formation of silicides in field-effect transistors (FETs). The silicide formation is a technique that forms compounds (silicides) of silicon with metals for the purpose of increasing the performance of the field-effect transistors. The formation of silicides reduces resistances in gates and source/drain regions to achieve the high-speed operation of the field-effect transistors. Examples of the metals that form silicides which are under consideration include nickel (Ni), cobalt (Co), and titanium (Ti). In particular, nickel is promising as the most suitable material for finer geometries.
The silicide formation is achieved by depositing a film of metal such as nickel on source/drain regions of a semiconductor wafer and then performing a heating treatment on the semiconductor wafer. At this time, when the heating treatment is performed for a long time, the silicide abnormally grows laterally (in a direction from the source/drain regions to the gate) to break through source/drain junctions, thereby giving rise to a problem such that leakage current increases rapidly. To overcome such a problem, it has been proposed, for example, in U.S. Patent Application Publication No. 2013/0078802 to perform a heating treatment in a short time by irradiating a surface of a semiconductor wafer with a metal film deposited thereon with a flash of light.
Unfortunately, it has turned out that the characteristics of an interface between the silicide and silicon of a base layer are degraded to cause an increase in resistance if only a flash heating treatment is performed in an extremely short time by merely irradiating a semiconductor wafer with a metal film deposited thereon with a flash of light as disclosed in U.S. Patent Application Publication No. 2013/0078802. Devices that will become much finer in geometries in the future are more subjected to the influence of oxygen because the silicides in the devices are reduced in thickness. It is also necessary to suppress the oxidation of the silicides after the formation of the silicides because the silicides themselves are easily susceptible to oxidation.
The degradation of the interface characteristics of the silicides and the oxidation of the silicides themselves are caused by the heating treatment in the presence of oxygen. Oxygen that causes these problems mainly includes residual oxygen in a chamber and oxygen adsorbed on the surface of the semiconductor wafer (mainly adsorbed in the form of water). In particular, oxygen remaining in the chamber during the flash heating treatment becomes a big factor in the increase in the thickness of a silicon oxide film. In general, semiconductor wafers are transported into and out of a chamber at ordinary pressure in a flash lamp annealer. Oxygen in the atmosphere flowing into the chamber during the transport remains in the chamber to increase the concentration of oxygen.